Multiple amps effect on initial voltage?

All of the amplifiers will be working with the same voltage input.  It is not the voltage that degrades with multiple connections, but some loss can occur in the signal through the various units and their circuitry, as well as the number of connections and wires used. 

The preamp level signal is a very low noise, low current output.

Read that statement in http://www.bcae1.com/audoutex.htm  You can also read the page on head units and this page:  http://www.bcae1.com/preoutv.htm  Read especially the last paragraph of this page.

You will see that because the signal current is low to begin with, multiple in/out connections can likely degrade it.  The quality of the amplifiers is key to this type of setup working well.  You don't want any filters of any type affecting the signal from one amp to another.  And you should connect to the amp that supplies the most critical components - the front mains - before the signal gets sent to the next amp.  Either the sub amp or rear speakers amp should go last in the chain.  But after you read that paragraph I referred you to, which includes, in part...

The most common (and least degrading) effect on the signal is the slight reduction in the signal level. This slight reduction is generally never noticed and is completely normal. A bigger problem appears when some piece of equipment does not have a purely resistive input circuit. This is generally from a ultrasonic filter on the input. These will cause the high frequency part of the signal to be attenuated more than the rest of the signal. Equipment with higher output impedances will have more high frequency roll off than those with a lower output impedance.

 ... you can possibly decide the order based on the various amplifier's specs.

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.